1. Field of the Invention
The present invention relates to matter identifying devices, and more particularly, to those that measure the optical absorption by matter within an evanescent wave field generated by total internal reflection within a low-loss optical cavity.
2. Related Art
Optical absorption spectroscopy is fundamentally important in chemical analysis, providing decisive quantitative and qualitative information. Such diagnostic capabilities find substantial utility in both research and industrial process environments. Therefore, an advancement in sensitivity, accuracy, or adaptability of the technique will have a significant impact.
Absorption is usually determined from measurement of a ratio of optical powers at a certain wavelength. Recently, a new technique, termed cavity ring-down spectroscopy (E. K. Wilson, C&E News, Feb. 19, 1996, p. 34, incorporated herein by reference), has been developed to determine absorption by gases, which utilizes a pulsed light source and an optical cavity. Typically, a light pulse from a laser source is injected into a cavity which is formed by two high-reflectivity mirrors. The lifetime of the pulse in the cavity is highly sensitive to cavity losses, including absorption by gases. Measurement of the pulse decay time or "ring-down" time in the cavity can thereby provide a direct measure of absorption. Cavity ring-down eliminates the adverse effects of light source fluctuation, since the measurement is acquired with a single pulse of light. The feasibility of this technique arises from recent technological advances in optical polishing, which permit the fabrication of extremely low-scatter-loss optics. If ordinary optics such as high-reflectivity mirrors (R.about.99%)are used, the pulse lifetime in the cavity is too short for the cavity ring-down strategy to provide a significant improvement in sensitivity, as compared to conventional absorption methods. However, with the advent of superpolishing, such as that described in N. J. Brown, Ann. Rev. Mater. Sci. 16, p. 371 (1986), incorporated herein by reference, mirrors with 99.99% reflectivity or better can be fabricated to construct low-loss optical cavities, thereby permitting ultra-high sensitivity to be routinely realized. The cavity ring-down technique has thereby become a viable form of optical absorption metrology, with trace analysis capabilities that greatly exceed conventional absorption methods.
A stable optical cavity used to measure the optical absorption of a material, as disclosed in copending application Ser. No. 08/962,170, is shown in FIG. 1. A three element cavity 5 is formed by two high-reflectivity concave mirrors 10, 12 with equal radii of curvature, and a Pellin-Broca prism 14 in a right-angle configuration. A light source 15 for injecting light, described throughout as a laser, is positioned adjacent mirror 10, and a photomultiplier 19 is positioned adjacent mirror 12. The Pellin-Broca prism 14 provides a total internal reflection with very high internal transmission for a light beam 16a that is polarized in the plane formed by the three element cavity 5, since an incident beam 16b and an exiting beam 16c traverse the prism faces at the Brewster's angle N.sub.B. By properly mounting the Pellin-Broca prism 14, the light beam will traverse the Pellin-Broca prism 14 at minimum deviation, which minimizes aberrations and beam translation with rotation about Brewster's angle N.sub.B. Since the total internal reflection occurs at a hypotenuse surface 14a of the Pellin-Broca prism 14, an evanescent wave 18 decays exponentially into the region external to the hypotenuse surface 14a. Absorbing materials (not shown) placed within the decay length of the evanescent wave 18 can thereby be sensitively probed through the change in the decay time of a laser pulse injected into cavity 5. This decay time is detected by photomultiplier 19 which senses a very small portion of the injected light which escapes through mirror or reflector 12. Cavity losses for the configuration shown in FIG. 1 are largely determined by surface roughness induced scattering, although stress-birefringence of the Pellin-Broca prism 14 may induce polarization scrambling.
The optical cavity shown in FIG. 1 includes high-reflectivity concave mirrors 10 and 12 located a distance from the prism 14. The mirrors 10 and 12 must be properly aligned with each other and the prism 14 in order for the cavity to operate properly.